1 1 mm. Gan Long Bao Tong was the currency in circulation during the Qianlong period. The most representative casting bureaus are Baoquan Bureau, Baowu Bureau, Baoshan Bureau, Baozhe Bureau and Baosu Bureau, all of which belong to the official coinage institutions with a coin diameter of 25 mm. At that time, there were many private coin-casting behaviors among the people. The diameter of non-standard coins is 1 1 mm, so the smallest diameter of Bao Tong in Gan Long is 1 1 mm.